1. Technical Field of the Invention
The present invention relates in general to the mobile communications field and, in particular, to a method for decorrelating background interference signals in a time-synchronized mobile communications system.
2. Description of Related Art
Synchronization in cellular communications systems is often implemented by the use of a special synchronization burst using known transmitted sequences. Time synchronization is then achieved by comparing the known transmitted sequence with the received sequence by correlation techniques, and using the correlation peak or other properties as a time reference. As such, the precision that can be obtained depends on the bandwidth available, and the noise or interference background against which the useful signal appears.
The precision of the time-synchronization process can be increased if multiple transmissions of the synchronization sequence can be demodulated at the receiver. Otherwise, somewhat degraded background interference conditions may be tolerated for a specified degree of lower precision. However, a prerequisite for obtaining higher receiver gain through the use of multiple measurements is that the measured signals not be identical. This requirement can be met if the synchronization signals appear against a noise or interference background that is decorrelated between adjacent measurements. Better measurements can also be obtained if the properties of the sources creating the interference change between measurements. However, in most practical situations, such source changes do not occur.
An important area of interest in the cellular communications field is the area related to time synchronization of bursts in time-division multiple access (TDMA) systems. During an ongoing call in a TDMA system, synchronization can be maintained by the use of training sequences included in each of the MS""s transmitted bursts. In that regard, in the higher capacity cellular systems, the interference background is created by transmissions from numerous MSs other than the one involved in the call. As such, these MSs transmit bursts with training sequences that are fixed for finite periods. However, call connections often utilize transmissions having a high degree of time stability. Consequently, for a particular desired signal, the same background interference signals can appear and be measured for a plurality of measurement periods, which degrades both timing and synchronization accuracy. In this context, the background interference is referred to as correlated.
A special application of the above-described time synchronization approaches is in the mobile positioning field for TDMA cellular systems. For example, if a MS""s transmissions containing synchronization sequences are measured at a plurality of base stations, the time of arrival (TOA) or time difference of arrival (TDOA) of the received signals can be used, along with certain timing information and information about the geographical locations of the base stations, in order to estimate the MS""s position.
Essentially, at a common measurement instant, numerous base stations measure the arrival time of a signal received from the MS whose position is to be determined. A central computing function converts the different arrival times of the MS""s signal to distances, and calculates the results to determine the position of the MS. The precision in this method is determined primarily by the exactness of the burst arrival time at each base station, with respect to a common time reference. Also, measuring the TOA or TDOA of the MS""s transmissions at as many base stations as possible improves the positioning accuracy, especially in strong multipath environments. As such, in order to be able to increase the number of base stations that can be used to make valid TOA or TDOA measurements from a MS""s transmissions, it is highly desirable to be able to utilize multiple transmissions and signal combining techniques in order to enhance measurement sensitivity and thus precision. However, a problem with the existing time-synchronization measurement approaches is that they provide no means for generating background signal levels that can enhance performance by combining the signals from multiple independent measurements of desired signal sequences. As described in detail below, the present invention successfully resolves this problem and other related problems.
In accordance with a preferred embodiment of the present invention, a method is provided for decorrelating background interference in a TDMA cellular system, in which a MS inserts a different training sequence in each successive transmitted burst. The constantly varying background interference signal environment that results can be decorrelated at the receiver, which enhances the reception of multiple independent MS transmissions, and thus increases the sensitivity and precision of time-synchronization measurements, such as, for example, TOA or TDOA measurements made for MS positioning purposes.
An important technical advantage of the present invention is that it provides a method for generating background signal levels that can enhance performance by combining the signals from multiple independent measurements of desired signal sequences.
Another important technical advantage of the present invention is that it provides a method for using multiple transmitted bursts to enhance measurement accuracy in a MS positioning system.
Yet another important technical advantage of the present invention is that it provides a method for increasing the sensitivity and precision of time-synchronization measurements, such as, for example, TOA or TDOA measurements made for MS positioning purposes.
Still another important technical advantage of the present invention is that it provides a method for decorrelating background interference in a time-synchronized mobile communications system.